This invention relates to electrolysis and particularly to depletion of impurities formed and dissolved in electrolyte.
If an electrolyte contains an electrolyzable impurity which has a lower decomposition voltage than the electrolyte, the impurity can be preferentially decomposed by electrolysis. As one example, sodium carbonate dissolved in molten sodium chloride can be decomposed without evolving chlorine gas. As another example, water dissolved in molten sodium hydroxide can be decomposed by electrolysis without decomposing the sodium hydroxide. Water is a decomposition product of molten alkali hydroxide electrolysis and is an impurity since it diminishes yield of alkali metal and current efficiency of an electrolysis cell.
The primary decomposition of molten sodium hydroxide is EQU 2NaOH.fwdarw.2Na+H.sub.2 O+1/2O.sub.2
with sodium metal forming on the cathode and dissolving in adjacent catholyte, with water forming on the anode and dissolving in adjacent anolyte, and with oxygen also forming on the anode but in a gaseous phase which separates from the anolyte. The dissolved decomposition products can diffuse into opposite portions of the electrolyte to result in the secondary reaction EQU Na+H.sub.2 O.fwdarw.NaOH+1/2O.sub.2
so that the two moles of sodium metal produced according to the primary decomposition are diminished to one mole thereby limiting current efficiency of conventional cells to 50% for the mole mobile of the alkali metal ions. But even if an ideal diaphragm or other means existed for preventing such diffusion, the formed water would consume current for the decomposition EQU H.sub.2 O.fwdarw.H.sub.2 +1/2O.sub.2.
Since that current flows at the higher voltage needed for the decomposition of sodium hydroxide, an irrecoverable loss of energy corresponds to the difference of decomposition voltage between the sodium hydroxide and the water.